The present invention relates to the simultaneous dual use of radiation, e.g. visible light, for both a conventional application, e.g., illumination, combined with the additional application of transmitting information without wires. The present invention further relates to electronic ballast circuits for electric discharge lamps, e.g., fluorescent lamps. The present invention further relates to the application of a time-varying, modulated current through the lamp to produce electronically detectable variations in the lamp light that are invisible to the human eye. The present invention further relates to coding information in variations in the lamp light for purposes of transmitting all kinds of information, including, but not limited to, digital data, audio, textual, and graphical signals. The present invention further relates to efficient coding schemes to maximize the bandwidth or information transfer capability of the optical data channel. Wide bandwidth and efficiency are critical for intranets or other wide area networks that could be carried on the lighting in an office or factory. The present invention further relates to efficient power electronic circuits capable of producing modulated currents in a lamp with high power efficiency, maximum data rate, and the possibility of incorporating needed safety features such as galvanic isolation. The present invention further relates to the construction of receivers for detection of the modulated information in the lamp light.
Over half of the artificial light produced in the United States comes from lamps in which an electric discharge through a gas is used to produce illumination (J. Waymouth, Electric Discharge Lamps, MIT Press, Cambridge, Mass., 1971). The prevalence of electric discharge (e.g., fluorescent) illumination has led us to develop ways to inexpensively use discharge lamps for communication. The basic idea of using lighting to send information as well as to provide illumination appears to have originated at least as early as 1975 (M. Dachs, xe2x80x9cOptical Communication Systemxe2x80x9d, U.S. Pat. No. 3,900,404, issued August 1975). Dachs discloses an analog amplitude-modulation (AM) scheme to modulate the arc current in a fluorescent lamp, the xe2x80x9ccarrierxe2x80x9d signal, with an audio information signal. A more recent patent, xe2x80x9cBoost-Mode Energization and Modulation Circuit for an Arc Lampxe2x80x9d, U.S. Pat. No. 5,550,434 to King et al., issued August 1996, discloses an updated electronic circuit that also provides for AM modulation of the arc current with an analog signal. Such techniques are generally undesirable for the direct transmission of data since, among other reasons, low frequency content in the data may lead to perceptible flicker in the light output, and the noise immunity of the overall transceiver system is not optimal. Techniques for encoding digital information have been described in U.S. Pat. No. 5,657,145, issued August 1997 to Smith, xe2x80x9cModulation and Coding for Transmission using Fluorescent Lamp Tubesxe2x80x9d, and U.S. Pat. No. 5,635,915 issued June 1997 to Gray, xe2x80x9cTransmission Systemxe2x80x9d, which employed either a pulsed AM or a phase modulation technique, respectively. Both techniques transmitted data at a rate that is on the same order of magnitude as that of the power-line frequency (50/60 Hz), i.e., relatively slowly compared to typical modern lamp arc frequencies in the range of 20,000 to 40,000 Hertz. Other communication schemes have also been proposed that do not use the lamp light as the carrier, but instead use the lamp fixture as an antenna for transmitting conventional radio wave or microwave signals. U.S. Pat. No. 5,424,859 issued June 1995 to Uehara, et al., xe2x80x9cTransceiver for Wireless In-Building Communication System [sic]xe2x80x9d, for example, discloses techniques for mounting a microwave antenna on the glass surface of fluorescent and incandescent lamps.
In T. Buffaloe, D. Jackson, S. Leeb, M. Schlecht, and R. Leeb, xe2x80x9cFiat Lux: A Fluorescent Lamp Transceiver,xe2x80x9d Applied Power Electronics Conference, Atlanta, Ga., June 1997 (xe2x80x9cBuffaloexe2x80x9d, which is incorporate here in its entirety by reference), the authors outlined the possibility of using pulse-code modulation to transmit data with a fluorescent lamp. This scheme made use of a tri-level pulse coding, which led to a ballast design with a relatively high-complexity compared to the architectures described in the present invention. Also, the associated receiver was more complicated, and unable to support the high data rates achievable with the present invention.
Previous efforts to use lighting for communication have not appreciated the need to select the coding scheme and control codes based on the data being transmitted. Rivollet (R. Rivollet et. al, xe2x80x9cSystem and Method for transmitting messagesxe2x80x9d,WO98/02846, Jan. 22, 1998 which is included here in its entirety by reference) teaches a very complex coding scheme to avoid flicker in which two hexadecimal characters (requiring 8 bits (time periods) total) are required to send a single decimal data character from the message. Rivollet does not have the capability to send alphabetical characters. Rivollet also shows no understanding of the effect that control codes and start codes can have on flicker. U.S. Pat. No. 5,838,116, issued November 1998 to Katyl, et. al., xe2x80x9cFluorescent Light Ballast with Information Transmission Circuitryxe2x80x9d (which is included herein in its entirety by reference), teaches that a simple pulse code modulation scheme will be sufficient since xe2x80x9cthe modulation occurs at a sufficiently high frequency so that it is imperceptible in the range of human vision.xe2x80x9d. However, as discussed above, Katyl does not understand or teach that the data must be analyzed and a proper coding method selected. Buffaloe uses a three-level coding scheme to attempt to avoid visible flicker. However, Buffaloe does not appreciate that the control codes and start codes also can create flicker. If the data stream were to contain a string of start codes using the scheme of Buffaloe to ensure reception of the start, the result would be visible flicker. Buffaloe does not teach or suggest the analysis of the data stream including the control codes and start codes in selecting the data coding scheme. None of these previous schemes involve analyzing the data to be transmitted including control and start codes and selecting the data coding scheme based upon the analysis of the nature of data.
The Applicants have invented a communication network based on frequency modulated radiation (e.g., visible light) that eliminates the disadvantages of the systems in the prior art. It enables higher power commercial scale lighting systems to be used to transmit the signal. It eliminates undesirable visual flicker in the system and so allows simultaneous continuous use of the lighting fixtures as lighting while also providing the medium for communication. It allows analog, digital or analog/digital data to be sent and received. It increases the bandwidth available to transmit data, and it enables a number of applications, such as multiple digital data streams, to be performed using a single lamp. Improvements made in the current invention can result in unprecedented performance advantages in the operation and implementation of lamp transceiver systems.
This invention is the first to propose establishing a transceiver system using any radiating transmitter with dual utility where the primary utility is any application, not just illumination but also possibly range finding, lane marking, or other applications, and the secondary utility is communication. This invention is the first to propose the transmission of bandlimited analog information such as audio signals by using frequency modulation, which enhances the noise immunity and available bandwidth over previous schemes while specifically avoiding sensory perceptible flicker in the transmission. It is the first to propose the efficient transmission of digital data using pulse code frequency modulation, and also the first to propose encoding digital bits in sidebands around the carrier frequency of the transmitter. It is the first to propose the use of a nonlinear detector in a dual-use network receiver to improve settling and detection time of pulse-coded data. These schemes for the transmission and reception of digital data substantially enhance the available data transmission rate in comparison to schemes in the prior art, again while elimination perceptible flicker. It is the first to disclose schemes for creating multiple data transmission channels using the same transmitter, and the first to propose a receiver in a xe2x80x9cdual-usexe2x80x9d network capable of selecting one channel from a spectrum of available choices. It is the first to propose a receiver with variable xe2x80x9clock-inxe2x80x9d or transmitter capture characteristics, allowing the tailoring of the behavior of the receiver as it locks on to different transmitters. This feature could be especially important for optimizing the receiver""s behavior in way-finding applications, and in environments with many different closely spaced transmitters, to ease the process of acquiring and holding a data channel between the transmitter and receiver.
In one aspect, the invention is apparatus for generating electromagnetic radiation in which the radiation has both a first and a second utility. The electromagnetic radiation is modulated to produce electronically detectable variations to achieve the second utility, the variations not affecting the first utility. In one embodiment, the second utility is transmission of information. In this embodiment, the electromagnetic radiation is visible light in which the first utility is illumination and the second utility is the sending of information, the variations in the visible light being invisible to the human eye. Suitable apparatus is a lamp which may, for example, be a fluorescent, cold cathode or a high-intensity discharge lamp. Any transmitter of radiated energy could be used, however, including light emitting diodes, lasers or radio wave antennas.
In another aspect, the invention is a lamp for generating visible light to provide illumination and to transmit information to a receiver in which the variations in the light as a result of the information transmission are undetectable to the human eye. The lamp includes a source of visible light and circuitry including a ballast for modulating the output of the source to send information by means selected from the group including analog FM, sideband encoded digital pulse code FM, discrete pulse code FM with two level coding, and any other orthogonal bit coding scheme. The circuitry may further include a rectifier for drawing power from an AC source and controlling the power to have substantially the same shape and phase, but possibly different amplitude, as the AC source to insure near-unity-power-factor operation. An inverter is connected to receive power from the rectifier to create a high frequency alternating wave form, the output of the inverter forming an input to the lamp. It is preferred that the inverter include means for varying the frequency of the voltage produced by the inverter. It is also preferred that the inverter is operated with zero-voltage or zero current switching.
In another aspect, the present invention is a method of designing and constructing of a system which, by design, does not flicker. The inventors have demonstrated that the visual observation of flickering in a light which is transmitting data is a function of the data being transmitted. As an example, a simple pulse code modulation system used to transmit digital information might not display visible flickering if the code being sent is alternating digital 1s and 0s. However, if the message is a string of 1s followed by string of 0s, flicker will be observed. On the other hand, if a two level Manchester type code is used to transmit the data, then flicker will not be observed even if the data does include a string of 1s followed by a string of 0s. Analyzing the nature of the data which will be transmitted enables the system designer to select appropriate hardware and a coding scheme to transmit the data without visible flicker. At the same time, a designer can avoid an unnecessarily complex coding scheme that has a slower data rate than could be achieved because some of the data carrying capability of the hardware is consumed in an unnecessarily complex code. If, for example, the analysis of the data to be transmitted concludes that there is not a string of 1s followed by a string of 0s or a string of 0s followed by a string of 1s, then the simple pulse code modulation scheme would be perfectly satisfactory and the data rate could be doubled (since each bit transmitted in Manchester code requires two changes in lamp frequency.) The inventors have determined that one must consider the nature of the data in selecting system hardware and a data coding scheme in order to avoid flicker.
It is not sufficient to consider only the data itself in the analysis; one must also consider control codes like message start codes. Control codes and start codes are an essential part of transmission of messages. The data must be analyzed and a proper coding method selected, because control codes and start codes also can create flicker.
A similar analysis of data can be useful when analog frequency modulation is used to transmit data. For example, for a light transmitting around a nominal 30 kHz carrier frequency with analog frequency modulation, ordinary radio talk i.e. band limited voice signals, will not cause visible flicker when the degree of modulation is 10 kHz (20 kHz to 40 khz) but when percussion music is transmitted, the result is flicker. This flicker can be eliminated by confining the degree of modulation to 5 kHz (25 kHz to 35 kHz).
In particular, the present invention pertains, in part, to electronic circuits capable of controlling and modulating the arc current in a lamp. The circuits include means to draw power from a direct or alternating (utility) source. The circuits further include means to control or limit the magnitude of the current flowing in a lamp or collection of lamps. The circuits further include means to vary the current in the lamp to encode information in the lamp light with no visible flicker.
By xe2x80x9clampxe2x80x9d as that term is used herein, it is meant a device that produces radiated transmissions, including, but not limited to, infra-red, visible, and ultra-violet light, in response to an input electrical current which flows in the lamp. A typical example is a fluorescent lamp, although other types, such as high-intensity discharge lamps, light emitting diodes, lasers, cathode ray tubes, particle beam emitters, liquid crystal displays, electroluminescent panels, klystrons, and masers are also intended. Emitters of other types of radiation, such as radio antennae for applications in RADAR sets, ultrasonic transducers, and mechanical fans (xe2x80x9cradiatingxe2x80x9d air or water for instance) are also intended. By xe2x80x9cballastxe2x80x9d as that term is used herein, it is meant a circuit that controls the amplitude, frequency, and phase of the current waveform in the lamp.
By xe2x80x9crectifierxe2x80x9d, as used herein, it is meant a circuit that takes as input a voltage waveform from a power source and produces a DC or predominantly DC output voltage waveform.
By xe2x80x9cinverterxe2x80x9d as used herein, it is meant a circuit that takes as input a low frequency or DC electrical voltage waveform from a power source. The inverter produces a high frequency voltage waveform that can be applied to the lamp, or a lamp in combination with other electrical components such as inductors or capacitors. The frequency and phase of this output voltage waveform can be controlled by the inverter.
By xe2x80x9cswitchxe2x80x9d as used herein, it is meant a device that can either block or permit the flow of electric current in response to a low-power-level control signal. Typical examples of a switch include a bipolar junction transistor, a MOSFET, or an insulated-gate bipolar junction transistor (IGBT).
By xe2x80x9cloadxe2x80x9d as used herein, it is meant a lamp or lamps, possibly in combination with other electrical components including inductors, capacitors, resistors, and transformers, which are added to ensure that proper and safe operating voltages and currents are, or can be by virtue of control actions taken by the inverter, applied to the lamp or lamps. Typically, the load is connected to the output of an inverter.
In one embodiment of the invention, a system that is capable of controlling the current in a discharge or fluorescent lamp is provided. A rectifier circuit is used to draw power from the AC utility. The current drawn from the AC utility by the rectifier circuit is actively controlled to have the same shape and phase, but possibly a different amplitude, as the AC utility voltage waveform, ensuring near-unity-power-factor operation. The power drawn from the AC utility is used to create a predominantly DC output voltage with little alternating or ripple voltage. This DC voltage serves as the input to an inverter circuit.
The inverter circuit draws power from the DC bus and creates a high frequency alternating waveform that can be applied to the lamp, or the lamp in combination with other electrical components including transformers, inductors, or capacitors. For example, the inverter can be used to apply an AC square wave to the primary of a transformer whose secondary is connected to a series combination of an inductor and a capacitor and lamp in parallel. The inverter circuit includes special means to vary the frequency of the voltage produced by the inverter circuit. The frequency can, for example, be varied to encode information in the output voltage waveform and, therefore, the light produced by the lamp. To maximize efficiency, the inverter is operated with zero-voltage switching. For example, switches are turned on only when the voltage across the switch is zero, ensuring a nearly lossless turn-on transition.
In another embodiment, the inverter circuit could be energized directly by a DC or low frequency alternating power source, eliminating the need for a rectifier circuit. This mode of operation is particularly attractive in environments, e.g., automobiles or other transportation systems, where DC power is available a priori. Again, the inverter circuit includes special means to vary the frequency of the voltage produced by the inverter circuit. The frequency can, for example, be varied to encode information in the output voltage waveform and, therefore, the light produced by the lamp. Again, to maximize efficiency, the inverter is operated with zero-voltage switching. For example, switches are turned on only when the voltage across the switch is zero, ensuring a nearly lossless turn-on transition.
In either case, a receiver can be constructed which remotely samples the lamp light from a distance and decodes the information in the light encoded by the ballast.